Australia's largest ever class action case, involving almost 30,000 people taking on the ANZ bank over fee-gouging, could come down to just one witness for each party to decide the winner.

In the first of a series of class actions planned against Australian banks, law firm Maurice Blackburn is alleging ANZ charged their customers excessive fees since 2006.

Maurice Blackburn is claiming around $50 million for fees the banks charged, including dishonour fees on bank accounts, as well as over limit fees and late payment fees on credit cards.

In a directions hearing in the Federal Court on Thursday, Justice Ray Finkelstein told counsel for Maurice Blackburn and the defendant ANZ he would prefer both parties picked their best witness to try the case.

"One case, one calculation, one fee - then hang your case on the result of that on the basis if you lose your best case then you lose the lot," he said.

Maurice Blackburn counsel Michael Lee said it seemed a sensible course to take, but ANZ counsel Alan Archibald SC believed it could be more complicated than that.

"It is an enormous and difficult case," Mr Archibald said.

He said the bank's systems, processes and costs had changed over time so cases would be very diverse.

"Even if you focus on one account the task seems to be very, very large," he said.

But Justice Finkelstein said he would like to see the class action "stand or fall" on the best case with the other claimants "bound along the way".

Outside the court, Maurice Blackburn chairman Bernard Murphy told reporters his counsel would consider the judge's remarks and try to break the case down to a handful of claimants.

"We are seeking to slice the case up into manageable sections, but the defendant appears to be saying it's like an octopus with tentacles everywhere," Mr Murphy said.

"We will be approaching the court with some sensible method of case management.

"This is a case about excessive and exorbitant bank fees which are ... unconscionable."

Justice Finkelstein set December 15-16 for the case to get under way.

Another 11 banks, including the Commonwealth, Westpac and National Australia Bank, are set to have similar actions taken against them by Maurice Blackburn.